Multi-stroke engine operation (e.g., varying the number of strokes in a combustion cycle) is one method that may improve fuel economy while maintaining reserve torque capacity for elevated load conditions. During Multi-stroke operation, selected strokes of each cylinder may not follow a typical four-stroke cycle for some conditions but rather perform extra compression stokes, for example. Operating an engine in this fashion can extend the torque interval, thereby lowering the average torque available from an engine. However, it can also increase fuel economy, at part load, by increasing the cylinder air amount and thermal efficiency. Further, the cylinder may resume four-stroke operation by simply eliminating any benign pumping strokes after a combustion event in the multi-stroke cylinder. Consequently, the ability to control the stroke count, e.g., 2-stroke, 4-stroke, 6-stroke, and/or 12-stroke, can enable cylinders on-demand may be used to improve fuel economy while retaining torque capacity.
One method to control intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 6,332,446. This method provides a way of controlling electromechanical valves in a cylinder that may reduce engine vibration in a cylinder that is operating in a cylinder deactivation mode or in a multi-stroke mode. The method controls pressure in the deactivated cylinders by adjusting intake and exhaust valve timing.
However, the inventors herein have recognized that the before-mentioned approach can have several disadvantages. Namely, the approach regulates pressure in a cylinder, via valves, after a combustion event. However, the above method can result in degraded airflow control during low load operation. For example, multi-stroke and deactivation may be used during low engine load conditions to increase fuel economy. However, low cylinder loads may reduce the inducted cylinder air amount that is combined with injected fuel to produce combustion pressure. Electromechanical valve constraints may make it difficult to control lower cylinder air amounts because the valves may have a minimum open time that is related to the valve inertia and to the actuator coil construction.
Such degraded air control may make it difficult for the before-mentioned method to regulate the combustion pressure and engine torque as suggested. Further, inaccuracies in the cylinder air amount may result in increased emissions and poor drivability. Also, the method may degrade cylinder combustion characteristics, such as burn rate or percent of mass fraction burned, since it may not affect cylinder charge motion, i.e., swirl and tumble, enough to compensate degraded combustion characteristics.